The high cost and limited availability of fish oil makes plant‐derived lipids attractive for aquafeed manufacturing, but replacing fish oil with these lipids can result in long‐chain polyunsaturated fatty acid (LC‐PUFA) deficiencies. Fatty acid metabolism, specifically the efficiency of LC‐PUFA utilization, may be influenced by the dietary saturated fatty acid (SFA) content versus that of C18 polyunsaturated fatty acids (PUFAs). We assessed the growth and tissue composition of Cobia Rachycentron canadum (55.3 ± 0.2 g initial weight [mean ± SE]; 10 fish/tank, 3 tanks/diet) fed diets (∼49% protein, ∼10% lipid) containing fish oil; 22:6(n‐3)–amended standard, partially hydrogenated, or fully hydrogenated soybean oil; and these same soybean oils supplemented with soybean lecithin for 8 weeks. Although survival (range = 97–100%), final weight (160–189 g), and feed conversion ratio (1.40–1.52) were unaffected by diet, differences were observed in weight gain (185–241%), specific growth rate (1.87–2.19% body weight/d), and feed intake (2.94–3.44% body weight/d). Significant effects of soybean oil type on final weight, weight gain, feed conversion ratio, specific growth rate, and feed intake were noted, with standard soybean oil generally outperforming the other soybean lipids when oil types were pooled across phospholipid supplementation treatments, whereas phospholipid supplementation had no significant effect on any of the performance measures. Differences in dietary fatty acid profile yielded differences in tissue composition. Feeding standard soybean oil resulted in the most greatly modified profiles, whereas the profiles of fish fed fully hydrogenated, completely saturated soybean oil were most similar to those of the fish oil–fed fish. The magnitude of profile change was greatest in the liver and fillet tissues and smallest in the eye and brain tissues. Although further research is necessary to demonstrate whether SFA‐rich lipids can effectively reduce the LC‐PUFA requirements of Cobia, it is clear that SFA‐rich oils offer a strategic advantage in minimizing the effects of fish oil replacement on tissue fatty acid profile.

Limited availability and high prices underscore the need to use fish oil more judiciously in aquafeeds. Most alternative lipids contain little to no n‐3 long‐chain (LC) polyunsaturated fatty acids (PUFAs). If alternative lipids are fed exclusively, growth performance may be impaired and tissue fatty acid composition may be altered in fish such as the Cobia Rachycentron canadum. Alternative lipids that are rich in saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs) may increase the efficiency of LC‐PUFA metabolism and may limit or attenuate the tissue LC‐PUFA loss associated with fish oil sparing. Cobias (weight [mean ± SE] = 77.4 ± 0.2 g) were fed diets containing either fish oil (control) or a 50/50 blend of fish oil and standard soybean oil, partially hydrogenated soybean oil, fully hydrogenated soybean oil, pork lard, or beef tallow. After 8 weeks, the feed conversion ratio (mean ± SE = 1.56 ± 0.04), weight gain (180 ± 6%), and specific growth rate (1.83 ± 0.04% of body weight/d) were equivalent among groups. Dietary fatty acid profiles influenced tissue composition; fatty acid profiles of control fish were more similar to those of fish that received higher levels of SFAs and MUFAs than to those of fish that were fed higher levels of C18 PUFAs. The SFA‐ and MUFA‐rich lipids, such as fully hydrogenated soybean oil and beef tallow, may be useful in reducing the fish oil quantities needed in Cobia feeds while maximizing fillet LC‐PUFA content.

Replacement of long‐chain (LC) polyunsaturated fatty acid (PUFA)‐rich fish oil with alternative lipids in aquafeeds typically reduces the LC‐PUFA content and associated nutritional value of farmed fish even if production performance is unaffected. Finishing can be used to augment tissue LC‐PUFA levels prior to harvest; however, the effectiveness of this strategy for use with the Cobia Rachycentron canadum is relatively unknown. For 8 weeks, Cobias (initial weight [mean ± SE] = 59.8 ± 0.2 g) were fed diets in which the supplemental lipid consisted of 100% fish oil; 100% beef tallow; a blend of 33% beef tallow and 67% fish oil; or a blend of 67% beef tallow and 33% fish oil. After the 8‐week grow‐out period, all treatment groups received the 100% fish oil feed for 6 weeks to simulate finishing. Differences in production performance were observed, but growth and growth efficiency were not reduced by inclusion of beef tallow in the grow‐out feeds. Prior to the finishing period, fatty acid profiles of fillet, liver, eye, and brain tissues varied considerably among treatments. As finishing progressed, tissue profiles converged on the profiles of fish that were fed the 100% fish oil diet exclusively. Specifically, saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs) declined, whereas LC‐PUFA levels varied relatively little during the trial. Results indicate that SFA‐ and MUFA‐rich alternative lipids like beef tallow minimize LC‐PUFA loss during grow‐out and that finishing feeds can be used to modify Cobia tissue profiles prior to harvest.